Heme catabolism in mammals will be studied by using solubilized heme oxygenase and synthetic hemins and will be compared with the results obtained with the membrane bound oxygenase. These studies are important in view of the fact that porphyrins and metalloporphyrins are increasingly being used as therapeutic, diagnostic and as phototherapeutic agents. The molecular properties of the isoforms of biliverdin reductase of liver will be studied in order to understand the mechanism of the interconversion among the isoforms which is triggered by the oxidative stress induced by chemicals which degrade liver hemo- proteins. The oxidation of the mitochondrial hemes c and a will also be examined to clarify if the oxidation of these hemins follows the same oxidative pathway as heme b. This is of interest to understand the turnover of the mitochondrial hemoproteins, which is still unknown. The chemical structure and properties of porphobilinogen oxygenase will be investigated. This enzyme oxidizes porphobilinogen (PBG) to oxopyrrolenines and diverts the former from its main physiological function as the precursor of heme. The balance of PBG-oxygenase vs PBG-deaminase (the enzyme which forms porphyrin) is one of the factors which keeps the balance of heme synthesis in erythropoietic tissues. This may be of significance to understand the imbalances of heme metabolism which lead to porphyrias. The activation mechanism of the PBG- oxygenase in the endoplasmic reticulum or erythropoietic tissues will also be examined. The mechanism of porphyrin biosynthesis from PBG by PBG-deaminase and uroporphyrinogen III cosynthase will be studied using 13C and 15N NMR techniques. This is the general mechanism by which porphyrins are formed in nature and its regulation affects the in vivo synthesis of all the hemoprotein and corrin derivatives. By using 13C and 15N enriched PBG the formation of the elusive intermediate of uroporphyrinogen III biosynthesis will be looked for. The biosynthesis of heme a will be investigated with the help of its possible metabolic precursor, 8-formyl-8-desmethylprotoporphyrin IX which was obtained by synthesis. The unraveling of the biosynthesis of heme a will help to understand the process of the in vivo synthesis and assembly of cytochrome c oxidase, a key component of the respiratory chain. In many biological oxidation processes catalyzed by hemoproteins, electrons are transferred through the porphyrin, while in others they are transferred through the metal. To examine the nature or these electron transfer processes as well as of the NH tautomerism reaction in porphyrins embedded in matrixes, solid state CPMAS NMR measurements will be performed on porphyrins and metalloporphyrins.